The famous ring nebula Messier 57 (M57, NGC 6720) is often regarded as
the prototype of a planetary nebula, and a showpiece in the northern
hemisphere summer sky.

Recent research has confirmed that it is, most probably, actually a ring
(torus) of bright light-emitting material surrounding its central star,
and not a spherical (or ellipsoidal) shell, thus coinciding with an early
assumption by John Herschel.
Viewed from this equatorial plane, it would thus more resemble the
Dumbbell Nebula M27 or the Little Dumbbell Nebula
M76 than its appearance we know from here:
We happen to view it from near one pole.

This is contrary to the belief expressed e.g. in Kenneth Glyn Jones' book.
There are even indications from investigations of deep observations such
as George Jacoby's deep photos
obtained at Kitt Peak National Observatory that the overall shape might be
more that of a cylinder viewed along the direction of the axis than that
of a ring, i.e., we are looking down a tunnel of gas ejected by a star at
the end of its nuclear-burning life.
Eventually, these observations have given evidence that the equatorial
ring or cylinder has lobe-shaped extensions in polar directions, similar
to those found in deep images of M76, but even more resembling other
planetaries like
NGC 6302,
see e.g. the review by Sun Kwok (2000).

The deep observations also show an extended halo of material extending
off to over 3.5 arc minutes (Hynes gives 216 arc seconds, quoting
Moreno & Lopez, 1987), remainders of
the star's earlier stellar winds. The halo was discovered in 1935 by
J.C. Duncan (Duncan, 1935).

Our color photo (taken with the 200-inch Hale telescope at Mt. Palomar)
shows that the material of the Ring is exposing a decreasing ionization
level with increasing distance from the 100,000 to 120,000 K hot central
star.
The innermost region appears dark as it emits merely UV radiation,
while in the inner visible ring, greenish forbidden light of ionized
oxygen and nitrogen dominates the color, and in the outer region, only
the red light of hydrogen can be excited.

The central star was discovered in 1800 by the German astronomer
Friedrich von Hahn (1742-1805),
with a 20-foot FL reflector. This object is a planet-sized white dwarf star,
which shines at about 15th magnitude.
It is the remainder of a sunlike star, probably once of more mass than our
sun, which has blown away its outer envelopes at the end of its Mira-like
phase of evolution. Now over 100,000 K hot, it will soon start to cool down,
shine as a white dwarf star for a while of several billions of years, and
then eventually end as a cold Black Dwarf.

As for most planetary nebulae, the distance to the Ring Nebula M57 is not
very wellknown. In case of this nebula, however, attempt was made to relate
its angular expansion rate of roughly 1 arc second per century with its
radial expansion velocity. These results, however, were based on wrong
assumptions of the geometry of this nebula, presuming a spherical shape.
Therefore, until recently, only rough estimates could be made, based on
various theoretical assumptions and models. The following distance values
have been given: 4,100 ly (K.M. Cudworth 1974; Mallas/Kreimer),
1,410 ly (Kenneth Glyn Jones), 2,000 to 2,500 ly (Vehrenberg),
2,000 ly (Sky Catalogue 2000.0), "more than 2,000 ly" (Murdin/Allen's
Catalogue of the Universe), 5,000 ly (Chartand/Wimmer's Skyguide),
3,000 ly (WIYN), and 1,000 to 2,000 ly (Sun Kwok, 2000).
A good value for the distance still needs to be determined (e.g., parallax by
Hubble Space Telescope), but recently improved CCD technics was used at the
US Naval Observatory (USNO) to determine a trigonometric parallax for the
central star of M57, yielding 2,300 ly
(Harris et.al. 1997, see also
STScI/Nasa, Jan 1999).

From the expansion rate of one arc second per century given above, the age of
the nebula can be roughly estimated under the assumption of constant expansion.
For its extension of 60x80 arc seconds, this yields a time of expansion of
about 6,000 to 8,000 years.

As most planetary nebulae, the Ring is much brighter visually at magnitude
8.8 than photographically at only 9.7 mag; a consequence of the fact that
most light is emitted in very few particular spectral lines (see the
discussion in our planetary nebulae page).
Assuming a distance of 2,300 lightyears, this corresponds to an absolute
magnitude of -0.3 visually (+0.5 photographically), or an intrinsic brightness
of about 50 to 100 times that of our Sun. Even the 14.7-mag central star, of
the size of a terrestrial planet, is only little fainter than our Sun with an
absolute magnitude of about +5 or 6. Its apparent dimension of 1.4 arc minutes
corresponds to a linear diameter of 0.9 lightyears (5.5 trillion miles or 8.8
trillion km, or 60,000 Astronomical Units), the halo extending out to a
diameter of 2.4 lightyears.

The mass of the nebular matter has been estimated at about 0.2 solar masses,
the density at about 10,000 ions per ccm (cm^3). Its chemical composition has
been determined as follows: On each Fluor (Fl) atom, the Ring Nebula contains
4.25 million atoms of Hydrogen (H), 337,500 Helium (He), 2,500 Oxygen (O),
1,250 Nitrogen (N), 375 Neon (Ne), 225 Sulfur (S), 30 Argon (Ar) and
9 Chlorine (Cl) atoms.
It is expanding at 20 to 30 km/s, and approaching us at 21 km/s.

M57 was the second planetary nebula to be discovered (in January 1779),
15 years after the first one, M27.
Antoine Darquier de Pellepoix (Darquier),
who discovered the Ring Nebula only a few days before
Charles Messier found and
cataloged it, described it as
"a dull nebula, but perfectly outlined; as large as Jupiter and looks
like a fading planet." This comparison to a planet may have influenced
William Herschel, who found the
objects of this type resembling the planet newly discovered by him, Uranus,
and introduced the name "Planetary Nebulae". Herschel described M57
as "a perforated nebula, or ring of stars;" this was the first mention of
the ring shape. Oddly, the inventor of the name "Planetary Nebula" did not
count this most prominent representative in this object class, but described
it as a "curiosity of the heavens", a peculiar object. Herschel also
identified some of the superimposed stars, and correctly assumed that
"none [of them] seems to belong to it."

M57 is very easy to locate as it is situated between Beta and Gamma Lyrae,
at about one-third the distance from Beta to Gamma. It can be seen with
binoculars as an almost stellar object, difficult to identify just because of
its small apparent diameter. In smaller amateur telescopes, the ring becomes
apparent at about 100 magnification, with a darker middle; a 12th-mag star is
east of the planetary nebula, about 1' of the center. If ever color is notable,
the Ring Nebula appears slightly greenish, not unexpected because most of its
light is emitted in few green spectral lines. Even in small scopes, a slight
ellipticity can be noted, with major axis in a position angle of about 60 deg.
With increasing aperture and under good condition, more and more detail becomes
visible, but even in large instruments, the central star will be apparent only
under exceptionally good conditions, or with the help of filters. In large
instruments, several very faint foreground or background stars can be glimpsed
within the nebula's extension under very good conditions.

Of the neighboring stars,
Beta Lyrae
(Sheliak) is a notable eclipsing binary, with components of spectral type
B7 and A8, varying between mag 3.4 and 4.4 with a period of 12.91 days. Gamma
Lyrae (Sulaphat, Arabic for "Tortoise") is a giant of spectral type B9 III and
mag 3.2 with a mag 12 companion at 13.8" distance in position angle 300 deg.
The 0.4' small and 14.4-mag faint galaxy IC 1296 is situated just 4' NW of M57
and can be found with large instruments.